A pro-ethanol Winnebago has been shadowing Ted Cruz’s appearances in Iowa to decry the candidate’s stance on the alternative fuel popular with Hawkeye State farmers who grow its compounds. (S.A. Miller/The Washington Times) more >

ALBIA, Iowa— Wherever Ted Cruz appears on the campaign trail in the last frantic days before the Iowa caucuses, chances are a colorful Winnebago is in hot pursuit.

As the Texas senator spoke to several hundred voters here Tuesday at Bogie’s Steakhouse, the RV festooned with a green cornfield and a “Ted Cruz can’t be trusted” sign stood vigil, a reminder of the corn-based fuel additive’s political clout as well as the unique character of Iowa politics.

“Caucusing for Cruz is caucusing against Iowa farmers,” declares a message on the RV painted in capital letters.

The federal ethanol mandate, which requires blending increasing amounts of ethanol into the nation’s gas supply, is of little interest to most voters outside the Hawkeye State, but it looms once again as a difference-maker in the pivotal first-in-the-nation caucuses.

The question of whether the $5 billion issue resonates with Iowans the way it once did comes with Mr. Cruz and billionaire businessman Donald Trump entering the final stretch locked in a virtual tie. A Quinnipiac University Poll released Tuesday found Mr. Trump leading Mr. Cruz by 31 to 29 percentage points among likely Republican caucusgoers.

“You play with ethanol in Iowa, you get burned,” said FiveThirtyEight political statistician Nate Silver on Twitter after Iowa Republican Party Chair Jeff Kaufmann introduced Mr. Trump at a Sunday rally.

For his part, Mr. Cruz has refused to back down from his opposition to ethanol subsidies, saying he wants a “fair and level playing field,” even as the issue embroils him in a feud with Republican Gov. Terry Branstad and sparks a furious last-minute ad battle leading up to the Monday vote.

Ethanol, a major market for Iowa corn, contributes an estimated $5 billion to the state’s economy and supports directly or indirectly about 47,000 jobs.

America’s Renewable Future, the group behind the Winnebago, launched another radio ad Tuesday “underlining Cruz’s hypocrisy.” At Cruz rallies on Tuesday, ARF aides handed out “report cards” giving all candidates a “good rating” except Mr. Cruz and Kentucky Sen. Rand Paul, who received a “bad rating.”

A poll commissioned by the American Council for Capital Formation found that half of the 700 Iowa voters surveyed said they “did not care much, or did not care at all, about the RFS and federal corn ethanol mandates.”

For 94 percent of those polled, the RFS and ethanol mandate was not among their top three issues, according to the survey released Friday.

“For as long as anyone can remember, conventional political wisdom dictated that candidates had no choice but to support ever-expanding corn ethanol mandates to win in Iowa,” said American Council for Capital Formation Executive Vice President George David Banks in a statement. “Unfortunately, they forgot to ask actual Iowans what they thought about it.”

The polling shows “not only aren’t folks in the nation’s largest corn-producing state paying particularly close attention to the back-and-forth over the RFS, they’re definitely not using it as some sort of litmus test in determining who to vote for.”

On the other hand, Frederick J. Boehmke, University of Iowa political science professor, said he wouldn’t be surprised if Mr. Cruz’s stance hurts him in the caucus vote. No candidate who has opposed the federal ethanol mandate since its inception in 2007 has won the Republican or Democratic caucus.

“Ethanol support is pretty important around here, especially for the agricultural economy,” Mr. Boehmke said in an email. “Branstad coming out so specifically to highlight that issue will likely cost Cruz some support.”

The popular Branstad made headlines last week when he said it would be a “big mistake” for Iowans to support Mr. Cruz, an unusual repudiation for a sitting governor about a candidate in his own party.

Supporters of Mr. Cruz, including Iowa Republican Rep. Steve King, have since noted that Mr. Branstad’s son Eric Branstad works for the pro-ethanol America’s Renewable Future, prompting the governor to say Monday that he’s “proud of what my son has done.”

“This is important to our state’s economy, and I want to make sure that the voters of Iowa are knowledgeable and well informed on all the candidates, and that they get out and vote in those caucuses, because it’s critical to our economy,” Mr. Branstad said during a Monday press conference.

While past presidential candidates, notably Arizona Sen. John McCain, have softened their anti-ethanol stances leading up to the caucuses, doing so would have been a stretch for Mr. Cruz, who hails from an oil-and-gas state and has sponsored legislation to drop ethanol from the RFS.

He received a boost Monday with a television ad buy from the ACCF attacking ethanol on environmental grounds. The ad cites a study showing that ethanol nearly doubles greenhouse gas emissions — as compared with gasoline — over 30 years.

The pro-Cruz Courageous Conservatives PAC defended the Republican in a radio and video ad released Monday that says “Cruz can’t be bought.”

“Ted Cruz opposes all energy subsidies, including oil subsidies, just like any conservative should. And Cruz had the guts to say it in Iowa,” the ad says. “That terrified Gov. Branstad and the ethanol lobby. They got Trump to promise bigger ethanol subsidies. And they announced that Cruz must be stopped at all costs.”

Mr. Kaufmann described ethanol as “an important issue out here,” but not one that would necessarily doom a candidate.

“I don’t think it’s an issue that will sink or allow one candidate to rise necessarily, but, I mean, it is a factor,” Mr. Kaufmann said Sunday in an interview on WNYM-AM in New York, as reported by The Hill. “If Sen. Cruz wants to explain his opinion, and I know he has an explanation for that, he can take it right to the people out here, and I’m guessing that he will.”